1. Field of the Invention
The present invention relates to an audio power amplifier, and more particularly to a multi-channel amplifier arranged to amplify high frequency components of an audio signal by an analog amplifying circuit and to amplify low frequency components by a class D amplifying circuit.
2. Description of the Prior Art
As an audio signal amplifying circuit, there are generally known a class A amplifying circuit which is designed so that a collector current flows at all times and a class B amplifying circuit designed so that the collector current flows for each half cycle when an alternating base input is applied. In an amplifier using such circuit as mentioned above, problems are that its power supply circuit becomes great both in size and weight, and that the power efficiency is poor which is especially prominent in an amplifier employing class A amplifying circuit. To cope with these problems, there has been provided an amplifier employing a so-called class D amplifying circuit to realize compact size, light weight and good power efficiency. An example of this prior art amplifier is shown in FIG. 1, in which an input audio signal to an input terminal 1 is supplied to a comparator 2 for comparison between said audio signal and a carrier signal having a sawtooth waveshape supplied from a clock pulse generator 3 to form a pulse-width-modulated signal. This pulse-width-modulated signal is supplied, via a driver circuit 4, to a switching circuit 5 which comprises transistors 6a and 6b of SEPP connection to thereby cause alternate on-off actions of these transistors 6a and 6b in accordance with the pulse width of said pulsewidth-modulated signal, so that the pulse-width-modulated signal is power-amplified through the switching transistors 6a and 6b with suitable source voltages .+-.V. The output signal of this switching circuit 5 is demodulated by an averaging low-pass filter 7, i.e. removal of the carrier signal from this output signal, to thereby obtain an audio output to drive a load 8 such as a speaker.
In such prior art amplifier as mentioned above, however, the above-stated advantages are accompanied by a disadvantage in that the fidelity of signal amplification is somewhat inferior. Accordingly, when it is desired to faithfully amplify the audio signal of the high frequency range, there is the necessity to set high the carrier frequency for the pulse-width modulation, for example, when it is intended to handle a single of a frequency up to 50 kHz, the carrier frequency is set at about 500 kHz. By so arranging, however, a switching transistor having a high cut-off frequency is required, and a further drawback is expected that there is produced spurious radiation of the carrier signal which adversely affects other electronic devices. The problem of this spurious radiation of carrier signal is a very troublesome one in view of the fact that this amplifier is a power amplifier. Furthermore, there could arise another problem of an increase in distortion caused by the leak of carriers.